The invention herein relates to thermal insulations. More particularly it relates to high density thermal insulations composed largely of calcium silicate hydrate.
Calcium silicate hydrate insulations have in the past been divided by industry practice into two groups: low density and high density. The low density or lightweight insulations generally have densities of less than 20 lb/cu. ft. ("pcf"; 0.3 g/cm.sup.3). Such materials are used primarily as short length pipe coverings and blocks for tank insulations and the like. They have little physical strength or impact resistance and must be used in settings where they will not be subjected to any physical impacts. In the past such lightweight materials were calcium silicate hydrates reinforced with asbestos fibers; a typical example of such materials is illustrated in U.S. Pat. No. 3,001,882. Subsequently asbestos-free calcium silicate hydrate lightweight insulations were developed, and typical examples are illustrated in U.S. Pat. Nos. 3,501,324 and 3,679,446. In most practical industrial uses the lightweight calcium silicate hydrate insulations have densities on the order of 9 to 15 pcf (0.14 to 0.22 g/cm.sup.3); a typical example is an asbestos-free calcium silicate hydrate insulation with a density of about 11 pcf (0.18 g/cm.sup.3) sold commercially by the Johns-Manville Corporation under the trademark THERMO-12.
Industry practice has heretofore defined high density calcium silicate hydrate insulations as those having densities of 20 pcf (0.32 g/cm.sup.3) or higher; commonly these have densities of 35 to 65 pcf (0.56 to 1.04 g/cm.sup.3). These high density materials offer not only good thermal insulation properties but also sufficient strength and durability such that they can be manufactured as large sheets and used for self supporting walls, oven linings, ship bulkheads and the like. Unlike the low density materials, the high density insulations have good nailing, cutting and screw holding characteristics, such that they can be handled in a manner similar to many other construction materials. A high density material of this type which established a very significant place in the market was an asbestos reinforced calcium silicate hydrate board having densities ranging from 23 to 65 pcf (0.37 to 1.04 g/cm.sup.3) and which was sold commercially by the Johns-Manville Corporation under the trademark MARINITE. Descriptions of such high density calcium silicate hydrate insulations will be found in U.S. Pat. Nos. 2,326,516 and 2,326,517.
In addition, calcium silicate insulations have been classified on the basis of the crystalline structure of the calcium silicate hydrate which makes up the insulation matrix. References have shown that the crystalline structure of the insulating calcium silicate hydrates can be varied among tobermorite, xonotlite and mixtures thereof, depending on the reaction conditions involved. See, for instance, U.S. Pat. No. 3,501,324; Kalousek et al, J.Am.Cer.Soc., 40, 7, 236-239 (July, 1957); and Flint et al, Research Paper RP-1147, J.Res.Natl.Bur.Stds., 21, 617-638 (November, 1938). U.S. Pat. No. 3,116,158 to W. C. Taylor describes insulations having tobermorite, xonotlite or mixed matrices, and also containing wollastonite as a reinforcing fiber. Taylor stresses, however, that if the matrix is tobermorite wollastonite must be used sparingly and only as a partial substitute for asbestos. In order to prevent slurry settling and resultant poor products, Taylor teaches that the fibrous component in the tobermorite slurry must be at least 40% by weight asbestos and in some cases must be up to 85% by weight asbestos. With a xonotlite matrix, however, all the asbestos may be replaced by wollastonite. Similarly, U.S. Pat. No. 3,001,882 also to Taylor describes the addition of 10% wollastonite to a matrix already containing 20% asbestos. U.S. Pat. No. 3,501,324 describes the addition of wollastonite to a xonotlite matrix as does U.S. Pat. No. 3,317,643. Other patents, such as U.S. Pat. No. 3,238,052 and British Pat. No. 984,112 mention wollastonite in other crystalline phases of calcium silicate materials. Wollastonite as the matrix itself is disclosed in U.S. Pat. No. 3,928,054.
While the high density asbestos reinforced calcium silicate hydrate thermal insulations have proved to be highly effective for many years, recent questions raised regarding the health aspects of asbestos fiber make it desirable to provide an asbestos-free high density calcium silicate hydrate insulation, which insulation will be comparable in thermal and physical properties to the prior art high density asbestos containing insulations.
In addition, it is desirable to have an asbestos-free tobermorite calcium silicate hydrate insulation, for tobermorite is significantly easier to form at moderate levels of temperature and pressure than xonotlite, as the above patents such as U.S. Pat. No. 3,501,324 evidence. Consequently if there could be full substitution of wollastonite for asbestos in the high density tobermorite products and one did not have to obtain yeilds of xonotlite to fully utilize wollastonite, high density calcium silicate hydrate reinforced product production would be significantly enhanced.